This invention relates to permanent magnet synchronous motors, and more specifically to an improved rotor construction therefor.
The synchronous operation of relatively small-diameter motors is required in some applications, for example the textile industry where it is necessary for a large number of motors to operate in precise synchronism. While in principle the construction of synchronous motors has long been known it is difficult and relatively expensive to construct small synchronous machines by heretofore conventional tenchiques--for instance, by providing salient poles with individual windings. Moreover, the high speeds required of such machines make a segmented or wire-wound rotor construction impractical.
In one approach to providing a rugged rotor which exhibits synchronous characteristics, induction motor rotors have been provided with flux barriers. In this manner the magnetic reluctance of the rotor is caused to vary with rotor orientation, which in turn causes the rotor to tend to "follow" the rotating magnetic field produced by the stator windings. This type of motor, termed a synchronous reluctance motor, is exemplified by U.S. Pat. Nos. 3,862,466--Hilgeman et al and 3,652,885--Honsinger. The flux barriers of these motors are provided by axial slots extending through the rotor laminations, which are filled with a nonmagnetic material such as cast aluminum. In addition, the motors are typically provided with a set of axial rotor bar slots about the peripheries thereof much in the manner of ordinary induction motors.
With the availability of high-strength permanent magnets, the cast flux barriers were partially replaced with magnets. The magnets served to produce magnetic poles in the rotor which interacted with stator flux to achieve synchronous motor operation. In effect, the permanent magnets take the place of electromagnetic windings of conventional synchronous motors. One example of such a construction is disclosed in U.S. Pat. No. 4,139,790--Steen.
The construction of rotors using permanent magnets to achieve synchronous operation is not without difficulty, however. Due to the high speeds encountered, the mechanical strength of the rotor laminations is of considerable importance and must be maintained. Further, pre-formed permanent magnets commonly are produced in the shape of rectangular bars which must be pressed into the rotor body, giving rise to exacting mechanical design considerations. Still further, the rare earth magnets which are commonly used in such rotors are very expensive which makes it desirable to limit the volume of magnetic material. At the same time it is desirable to provide the maximum magnetic area possible in order to produce strong magnetic poles. Finally, it is desirable to achieve all of these criteria while using magnets of a single, common size and shape.
It will therefore be appreciated that it would be highly desirable to provide an improved four-pole permanent magnet motor rotor which is mechanically strong, optimizes the use of magnetic material, and exhibits better performance characteristics than those previously known.
Accordingly, it is an object of the invention to provide an improved rotor for a permanent magnet motor.
Another object is to provide an economical permanent magnet rotor which exhibits improved performance characteristics.
Yet another object is to provide a four-pole permanent magnet rotor requiring less magnetic material than hereinbefore, yet with no degradation in performance.
Another object is to construct an improved four-pole permanent magnet motor utilizing only magnets of a single size and shape.